Touch sensing or touch position detection technology capable of providing a natural interface between an electronic system and user has found widespread applications in a variety of fields, for example, in mobile phones, personal digital assistants (PDAs), automatic teller machines (ATMs), game machines, medical devices, liquid crystal display (LCD) devices, light emitting diode (LED) devices, plasma display panel (PDP) devices, computing devices, and the like, where a user may input desired information and/or operate the electronic system through a touch sensing device associated with the electronic system. A touch sensing device typically includes a controller, a sensing circuit having a plurality of touch sensors and a network of control lines electrically connecting the plurality of touch sensors to the controller, and a touch panel associated with the plurality of touch sensors.
There are different types of touch sensing devices available for detection of a touch location. One is a resistive-type touch sensing device that includes two layers of transparent conductive material, such as a transparent conductive oxide, separated by a gap. When touched with sufficient force, one of the conductive layers flexes to make contact with the other conductive layer. The location of the contact point is detectable by a controller that senses the change in resistance at the contact point. In response, the controller performs a function, if any, associated with the contact point.
Another one is a capacitive-type touch sensing device. The capacitive-type touch sensing device can be classified into two types: an analog capacitive sensing device, which uses a contiguous resistive layer, and a projected capacitive sensing device, which uses patterned conductive layers (electrodes).
In a projected capacitive touch device, the touch sensor employs a series of patterned electrodes that are driven with a signal from a controller. Similarly, a location of the contact point can be derived from currents flowing through one or more corresponding electrodes toward the touch point responsive to the touch with sensing the capacitance induced by a user's finger. A finger touch to the sensor provides a capacitive couple from the conductive layer to the body. The location of the contact point is detectable by a controller that measures a change in a capacitively coupled electrical signal at the touch location. Accordingly, the controller performs a function, if any, associated with the touch location.
FIG. 16 shows schematically circuit diagrams of a conventional capacitive position detector. The output signal V(t) of the capacitive position detector has the following form:V(t)=VC·(1−e−t/τ),t≧0τ=ΣiRi·Ci,  (1)where VC is a supply voltage; Ri and Ci are resistance and capacitance value of the i-th capacitance sensor of the inducing capacitance detector, respectively. The output signal V(t) is plotted in FIG. 17. The capacitive position detector integrates the detector load and the capacitance of human finger as a compound variable to perform hierarchical signal triggering. Because the output signals for different compound capacitances have different steady-state time, the difference of threshold signals for different compound capacitances can be utilized to perform position detection.
However, for such a configuration of the capacitive position detector, it requires high resolution A/D conversions, which increases the complexity of the circuit of the detector, thereby increasing manufacturing costs. Moreover, if the detector load is much larger than the induced capacitance of a human finger, the sensitivity of the capacitive position detector will be limited. Accordingly, the capacitive position detector is not suitable in the large sized human-machine interface operation.
Therefore, a heretofore unaddressed need exists in the art to address the aforementioned deficiencies and inadequacies.